The present invention relates to a bicycle frame, particularly to a method of constructing a head tube junction of a bicycle frame which has a top tube and down tube jointed together at the head tube junction end.
Bicycles with frames fabricated from aluminium tubing have become increasingly popular. Unlike steel, aluminium cannot be brazed, so that joints between the tubes of most aluminium bicycle frames have to be welded. The joints between the tubes of the frame usually involve welding of each wall of one tube, ordinarily an edge at the end of the tube, to the surface of the wall of another tube. A critical joint in the manufacture of modern bicycle frames is the joint between the head tube, the top tube, and the down tube. The fork acts as a long lever arm and can exert significant amounts of stress on the head tube junction. The arrival of suspension bikes in the market place with stiff long-travel suspension forks have made the design of this junction even more critical.
Top tubes and down tubes have been getting bigger to achieve greater strength. At the same time, head tubes have been getting shorter to allow for the increased length of front suspension forks. The result is that many frames have top tubes and down tubes that overlap. In the prior art, the top or down tube is mitered to the head tube using hole saws and special fixtures to set up a drill press to position the tube for forming the edge cut. For example, the down tube has a certain diameter and forms a certain angle with the head tube. To make the end cut in the down tube for the welded joint with the head tube, the hole saw having a diameter equal to the diameter of the head tube is installed in a drill press, and the down tube is held in a fixture so that the axis of the hole saw is at the same angle to the down tube as the head tube is in the completed frame. The top tube is mitered to the head tube in the same manner.
The bicycle frames with overlapping down tubes and top tubes are traditionally constructed such that one of the top tube and down tube is mitered to the head tube only, and the other of the top tube and down tube is mitered to both the head tube and the one tube. This method of manufacture is limited to top tubes and down tubes with simple cross-sectional shapes. For example, when both the top tubes and down tubes are cylindrical, a portion of the wall of the top tube can be cut by a hole saw having a diameter equal to the diameter of the down tube at the same angle to the top tube as the down tube is in the completed frame.
However, the traditional method of mitering one of the top and down tubes to form the requested overlapping is impractical for tubes with non-round cross-sections. The welder is either left with a large gap to fill or else the miter itself is extremely complicated. As an example, U.S. Pat. No. 5,249,818, issued to Patterson on Oct. 5, 1993, describes a method of making a bicycle frame having the top and the down tubes overlapping at the head tube junction ends. A notch in the wall of the down tube is shaped to conform to and abut a portion of the surface of the wall of the top tube, and in the finished joint the weld is formed around the juncture between the notch and the wall portion of the tube that the notch abuts. The notch and other cut edges of the tubes are made by a cutting beam such as a laser beam. Relative movement is provided between the beam and the tube being cut such that the beam transverses a predetermined path over the wall of the tube and makes the desired cut. The relative movement includes a rotation of the tube about its axis and a simultaneous movement of the beam axially relative to the tube. This type of cutting process needs either expensive equipment if the cutting process is automatically completed or skillful operators if the cutting process is done manually.
Therefore, there is a need for a method of constructing a head tube conjunction of a bicycle frame which provides for optimum contact area and ease of manufacturing, especially for tubes with non-traditional cross-sectional shapes.
An object of the present invention is to provide a method of constructing a head tube junction of a bicycle frame which provides optimum contact area and ease of manufacturing, especially with tubes of non-traditional cross-sectional shapes.
Another object of the present invention is to provide a bicycle frame having a head tube junction with increased strength.
In accordance with one aspect of the present invention, there is a bicycle frame provided, comprising at least a head tube, a top tube, a down tube and a head tube junction formed therebetween. The top tube has one end cut at a first predetermined angle with respect to a longitudinal axis of the top tube. The end cut has an edge conforming to and abutting a portion of an external periphery of a wall of the head tube, whereby the top tube is mitered to the head tube at the first predetermined angle. Similarly, the down tube has one end cut at a second predetermined angle with respect to a longitudinal axis of the down tube. The cut end has an edge conforming to and abutting a portion of the external periphery of the wall of the head tube; thereby the down tube is mitered to the head tube at the second predetermined angle. An abutting surface is formed by cutting a portion of the wall of the top tube at the cut end thereof, and another abutting surface is formed by cutting a portion of the wall of the down tube at the cutting end thereof so that the abutting surfaces of the top and down tubes mate to form the head tube junction.
The head tube is preferably cylindrical, and each of the top and down tubes preferably tapers from a round cross-sectional shape to a rectangular cross-sectional shape at the end which forms the junction.
In accordance with another aspect of the present invention, a method is provided for constructing a head tube junction of a bicycle frame including at least one head tube, a top tube, and a down tube. The method comprises steps of cutting one end of the top and down tubes. The one end of the top tube is cut at a first predetermined angle with respect to a longitudinal axis of the top tube so that the top tube is enabled to be mitered to the head tube at the first predetermined angle. The one end of the down tube is cut at a second predetermined angle with respect to the longitudinal axis of the down tube so that the down tube is enabled to be mitered to the head tube at the second predetermined angle. The method comprises further steps of cutting a portion of the wall of the top tube at the cut end thereof to form an abutting surface, and cutting a portion of the wall of the down tube at the cut end thereof to form a mating abutting surface. Finally, the head tube junction is completed by welding at interfaces between the wall of the head tube and the respective cut ends of the top and down tubes, and between the abutting surfaces of the top and down tubes.
The portion of the wall of the top tube and down tube is preferably cut along a straight line at an angle with respect to the longitudinal axis of the respective top and down tubes.
By mitering the top tube and the down tube to each other, a number of benefits can be realized. The bicycle frame constructed in this manner has an increased strength because the jointed top tube and down tube sections act as a built-in gusset to greatly stiffen the head tube junction. The increased strength also results from the perfect contact between the top tube and the down tube through the entire length of the weld. The miter is greatly simplified and can in one form be accomplished with a straight cut and, therefore, the fit up for manufacturing is simpler than those used in the prior art. The weld between the top tube and down tube is at a preferred orientation, and the resulting joint is more aesthetically pleasing. Other advantages of the invention will be better understood with the description of a preferred embodiment.